1. Field of the Invention
The present invention relates to a measuring apparatus and a measuring method for performing measurement for an electronic circuit. More particularly, the present invention relates to a measuring apparatus that measures a jitter transfer function, a bit error rate and jitter tolerance of the electronic circuit.
2. Description of the Related Art
Jitter test is an important item to a serial communication device (serializer, deserializer or the like). For example, Recommendations and Requirements from International Telecommunication Union and Bellcore ((1) ITU-T, Recommendation G.958: Digital Line Systems Based on the Synchronous Digital Hierarchy for Use on Optical Fibre Cables, November 1994., (2) ITU-T, Recommendation O.172: Jitter and Wander Measuring Equipment for Digital Systems Which are Based on the Synchronous Digital Hierarchy (SDH), March 1999., and (3) Bellcore, Generic Requirements GR-1377-Core: SONET OC-192 Transport System Genetic Criteria, December 1998) define measurements of jitter tolerance, jitter generation and a jitter transfer function.
Therefore, the serial communication devices and the like have to satisfy the values described in the above specifications. For example, the jitter tolerance measurement of the deserializer is performed in the following manner. (a) A jitter (sinusoidal jitter) is incorporated into zero-crossings of an input bit stream. Then, (b) the deserializer samples the bit stream with the incorporated jitter at times in the vicinity of optimum sampling times so that the serial bit stream is output as parallel data. (c) One port is connected to a bit error rate measurement device so as to calculate a bit error rate. (d) The optimum sampling times have to be obtained from a recovered clock or a clock extracted from the data stream, in which the zero-crossings have jitter. Thus, it is apparent from the above that the jitter tolerance measurement is one of the most difficult measurements.
According to an well-known eye-diagram measurement, the performance of the communication device can be tested easily. FIG. 43 shows an eye diagram. The horizontal eye opening provides a peak-to-peak value of the timing jitter, while the vertical eye opening provides noise immunity or a signal-to-noise ratio. In the jitter tolerance measurement, however, the zero-crossings are caused to jitter by the timing jitter having a peak-to-peak value of 1 UI (Unit Interval, 1 UI is equal to the bit period Tb) or more. As a result, the eye-diagram measurement can merely measure a closed eye pattern. Therefore, it was difficult to apply the eye diagram to the jitter tolerance measurement.
FIG. 44 illustrates an exemplary arrangement for the jitter tolerance measurement of the deserializer. The jitter tolerance measurement is an extension of the bit error rate test. The deserializer performs serial-parallel conversion for the input serial bit stream, so that it outputs the resultant data as, for example, 16-bit recovered data. The instantaneous phase Δθ[nT] of the input bit stream to the deserializer under test is made to fluctuate by the sinusoidal jitter, where T is a data rate. A bit error rate tester provides a time delay to the output recovered clock so as to obtain the optimum timing, and samples the output recovered data. By comparing the sampled values of the recovered data with expected values corresponding thereto, the bit error rate of the parallel data is tested. However, since the output recovered clock is extracted from the serial bit stream in which edges fluctuate, it becomes difficult to sample the output recovered data at the optimum sampling times when the amount of the incorporated jitter is larger. On the other hand, according to the method in which the clock is extracted from the recovered data stream, the bit error rate tester has to include a high-performance clock recovery unit. This is because the jitter tolerance measurement of the deserializer requires the clock recovery unit which has larger jitter tolerance than that of the clock recovery unit included in the deserializer under test.
In other words, in the jitter tolerance measurement using the bit error rate tester, it is likely to underestimate the jitter tolerance. Therefore, in order to perform the measurement with excellent reproducibility, high measurement skill or know-how is required.
Moreover, in the jitter tolerance measurement, while the incorporated jitter amount is increased with the fixed jitter frequency fJ, the minimum incorporated jitter amount that causes generation of the bit error rate is obtained. For example, in order to perform the bit error rate test for a 2.5 Gbps serial communication device, 1-sec bit error rate test is performed by using a pseudo-random binary sequence having a pattern length of 215−1. Therefore, in order to change the incorporated jitter amount 20 times and measure the jitter tolerance for each of 20 types of incorporated jitter amount, the test time of 20 sec is required.
Timing degradation of the input bit stream increases the bit error rate as well as amplitude degradation. The timing degradation corresponds to the horizontal eye opening in the eye-diagram measurement, while the amplitude degradation corresponds to the vertical eye opening. Therefore, by measuring the degrees of the timing degradation and amplitude degradation, the bit error rate can be calculated. Please note that the jitter tolerance measurement corresponds to the horizontal eye opening in the eye-diagram measurement. For example, degradation of the amplitude of a received signal of ΔA=10% corresponds to the reduction of the signal-to-noise ratio of 20log10(100−10)/100=0.9 dB. Therefore, the bit error rate increases by 0.9 dB. As for the timing degradation ΔT, the similar calculation can be performed. Please note that the % value of the ratio and the dB value are relative values, not absolute values. Thus, in order to obtain an accurate value of the bit error rate, calibration is required.
John E. Gersbach, Ilya I. Novof, Joseph K. Lee, “Fast Communication Link Bit Error Rate Estimator,” U.S. Pat. No. 5,418,789, May 23, 1995 discloses the following definition.
                    BER        =                  10                                    -              κ                                      (                                                                    Δ                    ⁢                                                                                  ⁢                    T                                    T                                +                                                      Δ                    ⁢                                                                                  ⁢                    A                                    A                                            )                                                          (        1        )            
In the above patent, an instantaneous bit error rate is calculated from ΔA, ΔT, a local clock period T and the maximum value A of the samples at the optimum sampling times. However, the invention disclosed in the aforementioned patent merely provides a method for estimating the bit error rate by measuring the timing degradation by a Gaussian noise jitter.
The invention disclosed in the aforementioned patent obtains a histogram of data edges, performs a threshold operation and obtains ΔT. This operation is effective to the Gaussian noise jitter having a single peak. FIG. 45 compares a probability density function distribution of the Gaussian noise jitter and that of the sinusoidal jitter. The sinusoidal jitter used in the jitter tolerance test has two peaks at both ends of the distribution. Therefore, ΔT cannot be obtained only by performing the simple threshold operation. Moreover, in the jitter tolerance measurement, the zero-crossings are caused to jitter by the timing jitter of 1 UIPP or more. As a result, the histogram has the distribution in which the probability density functions of adjacent edges overlap each other, as shown in FIG. 45. From such a histogram, it is difficult to obtain ΔT. In addition, it is known that this histogram operation cannot secure a sufficient measurement precision unless about 10000 samples are obtained. Therefore, it is hard to reduce the measurement time. Moreover, K in the above expression does not have an ideal value. Therefore, by calibrating the instantaneous bit error rate with the actual bit error rate, the initial value for K has to be given. Also, a correction value ΔK has to be calculated from the difference between the long-term mean value of the instantaneous bit error rate and the actual bit error rate. In the conventional measurement, it was demanded that the difficulty in the jitter tolerance measurement was solved. Especially, it is demanded to reduce the test time.